The invention relates to an optical element that affects the polarization of light rays. The optical element has a thickness profile and consists or comprises of an optically active crystal with an optical axis.
In the continuing effort to achieve structures of finer resolution in the field of microlithography, there is a parallel pursuit of substantially three guiding concepts. The first of these is to provide projection objectives of very high numerical aperture. Second is the constant trend towards shorter wavelengths, for example 248 nm, 193 nm, or 157 nm. Finally, there is the concept of increasing the achievable resolution by introducing an immersion medium of a high refractive index into the space between the last optical element of the projection objective and the light-sensitive substrate. The latter technique is referred to as immersion lithography.
In an optical system that is illuminated with light of a defined polarization, the s- and p-component of the electrical field vector, in accordance with Fresnel's equations, are subject to respectively different degrees of reflection and refraction at the interface of two media with different refractive indices. In this context and hereinafter, the polarization component that oscillates parallel to the plane of incidence of a light ray is referred to as p-component, while the polarization component that oscillates perpendicular to the plane of incidence of a light ray is referred to as s-component. The different degrees of reflection and refraction that occur in the s-component in comparison to the p-component have a significant detrimental effect on the imaging process.
This problem can be avoided with a special distribution of the polarization where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the optical system have an approximately radial orientation relative to the optical axis. A polarization distribution of this kind will hereinafter be referred to as radial polarization. If a bundle of light rays that are radially polarized in accordance with the foregoing definition meets an interface between two media of different refractive indices in a field plane of an objective, only the p-component of the electrical field vector will be present, so that the aforementioned detrimental effect on the imaging quality is reduced considerably.
In analogy to the foregoing concept, one could also choose a polarization distribution where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the system have an orientation that is perpendicular to the radius originating from the optical axis. A polarization distribution of this type will hereinafter be referred to as tangential polarization. If a bundle of light rays that are tangentially polarized in accordance with this definition meets an interface between two media of different refractive indices, only the s-component of the electrical field vector will be present so that, as in the preceding case, there will be uniformity in the reflection and refraction occurring in a field plane.
Providing an illumination with either tangential or radial polarization in a pupil plane is of high importance in particular when putting the aforementioned concept of immersion lithography into practice, because of the considerable negative effects on the state of polarization that are to be expected based on the differences in the refractive index and the strongly oblique angles of incidence at the respective interfaces from the last optical element of the projection objective to the immersion medium and from the immersion medium to the coated light-sensitive substrate.
U.S. Pat. No. 6,191,880 B1 discloses an optical arrangement for generating an approximately radial polarization. The arrangement includes among other things a raster of half-wave plates whose respective directions of preference are oriented so that when linearly polarized light passes through the raster arrangement, the plane of oscillation is rotated into the direction of a radius originating from the optical axis. However, because the raster arrangement is produced by joining a large number of individually oriented half-wave plates, it is expensive to produce. Furthermore, the change in the direction of the polarization is constant within the area of each individual half-wave plate whose diameter is typically between 10 and 20 mm, so that no continuous radial polarization can be produced through this concept.
A birefringent element of crystalline quartz with an irregularly varying thickness is proposed in DE 198 07 120 A1 for the compensation of local aberrations of a defined state of polarization in an optical system. However, the variation in thickness in a birefringent element of this type leads to locally different states of polarization. In particular, the linear state of polarization is, as a rule, not preserved in an arrangement of this type.